Lameness is not always a result of a painful leg.     If pain is the cause of lameness then it may originate  from damage to  muscles, tendons, ligaments, bones, cartilage or it may result from a sliver, a broken nail or foriegn material between the food pads.   However lameness may  result from non painful causes.   Neurological damage to  the brain, spinal cord or peripheral nerves can effect motor nerve fibers that stimulate muscles to contract.  Neurologic damage may also cause loss of communication between the brain and back legs.  When the brain looses awareness of where the limb is in space an abnormal gait resutls.  Neurologic damage may also result in muscular weakness.  Such weakness in one limb will cause an abnormal gait to occur. Finally muscular weakness and lameness may also result from disuse atrophy.   This is particularly true with older arthritic animals

The point I am making is that diagnosiing a pet's lameness is often not cut and dry.    Neurologic damage must always be considered as a complicating factor